Epithelial moulds on the shells of the early Palaeozoic brachiopod Lingulella

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Abstract
A fine hexagonal network of microscopic ridges was discovered on the internal shell surface of the cosmopolitan Lower Paleozoic inarticulate genus Lingulella. The micro-ornamentation has been recognized on all well-preserved specimens examined, from Cambrian and Ordovician successions in Ireland, North American, UK, Spitzbergen and China. Examination of the mantle of the related Recent genus Lingula revealed that the outer epithelial cells are arranged in a hexagonal close-packed pattern comparable in size and form to the micro-ornamentation in the fossil shells. This fact, along with the discovery of a polygonal pattern on some organic layers in the Lingula shell, suggests that the micro-ornamentation in fossil Lingulella is the mold of the outer epithelium responsible for shell secretion. No trace of epithelial molds has yet been found in specimens of Lingula from younger Paleozoic successions. This difference may prove to be a useful diagnostic feature in distinguishing linguloid inarticulates from one another.

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